This invention relates to active matrix display devices, and relates in particular to the circuitry used for providing drive signals to the pixels of the display.
Active matrix display devices typically comprise an array of pixels arranged in rows and columns. Each row of pixels shares a row conductor which connects to the gates of the thin film transistors of the pixels in the row. Each column of pixels shares a column conductor, to which pixel drive signals are provided. The signal on the row conductor determines whether the transistor is turned on or off, and when the transistor is turned on, by a high voltage pulse on the row conductor, a signal from the column conductor is allowed to pass on to an area of liquid crystal material, thereby altering the light transmission characteristics of the material. An additional storage capacitor may be provided as part of the pixel configuration to enable a voltage to be maintained on the liquid crystal material even after removal of the row electrode pulse. U.S. Pat. No. 5,130,829 discloses in more detail the design of an active matrix display device.
The frame (field) period for active matrix display devices requires a row of pixels to be addressed in a short period of time, and this in turn imposes a requirement on the current driving capabilities of the transistor in order to charge or discharge the liquid crystal material to the desired voltage level. In order to meet these current requirements, the gate voltage supplied to the thin film transistor needs to fluctuate between values separated by approximately 30 volts. For example, the transistor may be turned off by applying a gate voltage of around xe2x88x9210 volts, or even lower, (with respect to the source) whereas a voltage of around 20 volts, or even higher, may be required to bias the transistor sufficiently to provide the required source-drain current to charge or discharge the liquid crystal material sufficiently rapidly.
The requirement for large voltage swings in the row conductors requires the row driver circuitry to be implemented using high voltage components.
The voltages provided on the column conductors typically vary by approximately 10 volts, which represents the difference between the drive signals required to drive the liquid crystal material between white and black states. Various drive schemes have been proposed enabling the voltage swing on the column conductors to be reduced, so that lower voltage components may be used in the column driver circuitry. In the so-called xe2x80x9ccommon electrode drive schemexe2x80x9d, the common electrode, connected to the full liquid crystal material layer, is driven to an oscillating voltage. The so-called xe2x80x9cfour-level drive schemexe2x80x9d uses more complicated row electrode waveforms in order to reduce the voltage swing on the column conductors, using capacitive coupling effects.
These drive schemes enable lower voltage components to be used for the column driver circuitry. However, there is still a significant amount of complexity and power inefficiency in the column driver circuits. Each row is addressed in turn, and during the row address period of any one row, pixel signals are provided to each column. In the past, each column would be provided with a buffer for holding a pixel in the column to a drive signal level for the full duration of the row address period. This large number of buffers results in high power consumption.
There have been proposals to provide a multiplexing scheme, in which a buffer is shared between a group of columns. The output of the buffer is switched in turn to the columns of the group. When the buffer is providing a signal to one column, it is isolated from the other columns by a switch. Multiplexing is possible because the line time of the display is significantly greater than the time required to charge a column to the required voltage. In small displays for mobile applications, the line time may be in excess of 150 xcexcs whereas the time required to charge a column is typically less than 10 xcexcs.
Once the column has been charged to the required voltage, and after the end of the application of the required voltage to the column, charge transfer takes place between the charged column capacitance and the pixel capacitance. The column capacitance may be around 30 times larger than the column capacitance, so that the charge transfer to the pixel results in only a small voltage change. However, this charge transfer enables the pixel to be charged using a short column address pulse, despite the longer time constant of the pixel (resulting from the high TFT resistance).
A problem with this multiplexing approach is that there is cross talk between the columns within the group, particularly as all but one of the columns of the group are effectively floating at any point in time, and are therefore susceptible to signal level fluctuations. During the row address period, the TFTs of all pixels in the row are switched on (and indeed this enables the charge transfer to take place between the column capacitance and the pixel), so that any signal fluctuations on the column conductors as a result of cross talk are passed onto the pixels.
According to a first aspect of the invention, there is provided a display device comprising an array of liquid crystal pixels arranged in rows and columns, wherein each column of pixels shares a column conductor to which pixel drive signals are provided, wherein column address circuitry is provided for generating the pixel drive signals, the column address circuitry comprising a plurality of multiplexing switching arrangements, each for providing drive signals to a plurality of columns in turn, wherein each multiplexing switching arrangement is associated with two buffers for providing selected pixel drive signals, wherein the two buffers provide respective pixel drive signals simultaneously to two adjacent columns, such that the pixel drive signal for one column starts before the end of the pixel drive signal for the column driven previously, and ends after the end of the pixel drive signal for the column driven previously.
The invention provides a multiplexing scheme which enables a reduction in the number of buffers required but which reduces the cross talk between column signals for adjacent columns within the group of columns shared by each multiplexing arrangement. This is achieved by ensuring that any capacitive coupling between first and second columns is charged to a static level before the signal on one of the columns is switched off. Thus, one column is only switched off after the next column has been addressed, so that any capacitive coupling between one column and the next is charged to a static level, and the signal on the next column no longer has any influence on the previous column.
Preferably, the apparatus further comprises circuitry for generating all possible pixel drive signals and a switching matrix for switching selected drive signals to the two buffers of each multiplexing switching arrangement. The switching matrix may receive digital image data and analogue pixel drive signals, and select the appropriate analogue pixel drive signal for each buffer based on the digital image data.
Each column may be provided with pixel drive signals twice within each row address period. This allows charge redistribution with the various capacitive elements of the columns after the first set of pixel drive signals, and then enables the second set of pixel drive signals to provide more accurate pixel control.
Each pixel preferably comprises a thin film transistor switching device and a liquid crystal cell, wherein each row of pixels share a row conductor which connects to the gates of the thin film transistors of the pixels in the row, and wherein row driver circuitry provides row address signals for controlling the switching of the transistors of the pixels of the row.
According to a second aspect of the invention, there is provided a method of providing pixel drive signals to a display device comprising an array of liquid crystal pixels arranged in rows and columns, the columns being divided into groups, each group sharing a multiplexing switching arrangement and two buffers for providing selected pixel drive signals, the method comprising, for each group of columns, applying pixel drive signals to all columns in the group in a cyclical manner, wherein the one column is provided with a pixel drive signal by one buffer before the pixel drive signal for the preceding column in the cycle, provided by the other buffer, is finished.
This method implements the drive scheme discussed above. At the end of the pixel drive signal to one column from each buffer, that buffer is used to apply a pixel drive signal to a column two ahead of the one column in the cycle. This results in a continuous cycle.
One multiplexing arrangement may address the columns of the respective group in a first order, and an adjacent multiplexing arrangement may address the columns of the respective group in a second order, such that columns in one group adjacent columns in the other group are addressed at substantially the same time. This smoothes errors across the display dependent on the specific timing of the address signals for different columns.
The invention also provides column address circuitry for driving the columns of a liquid crystal display, comprising a plurality of multiplexing switching arrangements, each for providing drive signals to a plurality of columns in turn, wherein each multiplexing switching arrangement is associated with two buffers for providing selected pixel drive signals, wherein the two buffers provide respective pixel drive signals simultaneously to two adjacent columns, such that the pixel drive signal for one column starts before the end of the pixel drive signal for the column driven previously, and ends after the end of the pixel drive signal for the column driven previously.